Resistance determination with increased sensitivity for temperature control of heated automotive components

ABSTRACT

Electrical resistance of a heated component is determined for temperature control and monitoring. A voltage-representation signal, which is proportional to a voltage across a heater component, is received. A magnified voltage-representation signal is generated by determining a difference between a voltage baseline offset, which represents a minimum operating voltage of the heater component, and the voltage-representation signal. A current-representation signal, which is proportional to an electrical current passing through the heater component is received. A magnified current-representation signal is generated by determining a difference between a current baseline offset, which represents a minimum operating current of the heater component, and the current-representation signal. The magnified current-representation signal is modulated to generate a resistance-representation signal that is proportional to an amount of modulation that makes the magnified current-representation signal approximately equal to the magnified voltage-representation signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following 5 U.S. provisional patentapplications:

Using Resistance Equivalent to Estimate Temperature of a Fuel-InjectorHeater, invented by Perry Czimmek, Mike Hornby, and Doug Cosby, filed onthe same day as this provisional patent application, and identified byAttorney Docket Number 2012P01913US.

Tuned Power Amplifier With Loaded Choke For Inductively Heated FuelInjector, invented by Perry Czimmek, filed on the same day as thisprovisional patent application, and identified by Attorney Docket Number2012P01914US.

Tuned Power Amplifier with Multiple Loaded Chokes for Inductively HeatedFuel Injectors, invented by Perry Czimmek, filed on the same day as thisprovisional patent application, and identified by Attorney Docket Number2012P01915US.

Using Resistance Equivalent to Estimate Heater Temperature of an ExhaustGas After-Treatment Component, invented by Perry Czimmek, Mike Hornby,and Doug Cosby, filed on the same day as this provisional patentapplication, and identified by Attorney Docket Number 2012P02060US.

Resistance Determination For Temperature Control Of Heated AutomotiveComponents, invented by Perry Czimmek, filed on the same day as thisprovisional patent application, and identified by Attorney Docket Number2012P02175US.

BACKGROUND

Embodiments of the invention relate generally to power electronics forautomotive heaters and more particularly for control of heaters forvariable spray fuel injectors or electronic catalysts.

There is a continued need for improving the emissions quality ofinternal combustion engines. At the same time, there is pressure tominimize engine crank times and time from key-on to drive-away, whilemaintaining maximum fuel economy. These pressures apply to enginesfueled with alternative fuels, such as ethanol, as well as to thosefueled with gasoline.

During cold temperature engine start, the conventional spark ignitioninternal combustion engine is characterized by high hydrocarbonemissions and poor fuel ignition and combustibility. Unless the engineis already at a high temperature after stop and hot-soak, the crank timemay be excessive, or the engine may not start at all. At higher speedsand loads, the operating temperature increases and fuel atomization andmixing improve.

During an actual engine cold start, the enrichment necessary toaccomplish the start leaves an off-stoichiometric fueling thatmaterializes as high tail-pipe hydrocarbon emissions. The worstemissions are during the first few minutes of engine operation, afterwhich the catalyst and engine approach operating temperature. Regardingethanol fueled vehicles, as the ethanol percentage fraction of the fuelincreases to 100%, the ability to cold start becomes increasinglydiminished, leading some manufacturers to include a dual fuel system inwhich engine start is fueled with conventional gasoline, and enginerunning is fueled with the ethanol grade. Such systems are expensive andredundant.

Another solution to cold start emissions and starting difficulty at lowtemperature is to pre-heat the fuel to a temperature where the fuelvaporizes quickly, or vaporizes immediately (“flash boils”), whenreleased to manifold or atmospheric pressure. Pre-heating the fuelreplicates a hot engine as far as fuel state is considered.

A number of pre-heating methods have been proposed, most of whichinvolve preheating in a fuel injector. Fuel injectors are widely usedfor metering fuel into the intake manifold or cylinders of automotiveengines. Fuel injectors typically comprise a housing containing a volumeof pressurized fuel, a fuel inlet portion, a nozzle portion containing aneedle valve, and an electromechanical actuator such as anelectromagnetic solenoid, a piezoelectric actuator, or another mechanismfor actuating the needle valve. When the needle valve is actuated, thepressurized fuel sprays out through an orifice in the valve seat andinto the engine.

One technique that has been used in preheating fuel is to resistivelyheat metallic elements comprising the fuel injector with a time-varyingor steady state electrical current. The electrical energy is convertedto heat inside a component suitable in geometry and material to beheated by the Joule or Ohm losses that are caused by the flow of currentthrough that component.

The heated fuel injector is useful not only in solving theabove-described problems associated with gasoline systems, but is alsouseful in pre-heating ethanol grade fuels to accomplish successfulstarting without a redundant gasoline fuel system.

Because the heating technique uses an electrical current, the systemincludes electronics for providing an appropriate excitation to thecomponent in the fuel injector. This excitation may include controllingthe electrical energy and determining when that electrical energy isapplied.

Conventional resistive heating is accomplished open-loop, or withoutcontrol of electrical energy based on a temperature. A remote thermostator computational model may be incorporated to provide some control toprevent a runaway temperature event and damage to the fuel injector.More sophisticated methods may monitor the current through the heater toestimate the temperature or direct thermocouple, positive/negativetemperature coefficient sensor, or other means for determining thetemperature for a more precise regulation of injector heatertemperature.

The metallic component that is heated will have a positive temperaturecoefficient of resistance to electrical current (i.e., its electricalresistance will increase as its temperature increases). Ideally, knowingthe initial resistance and final resistance would allow the temperatureof the component to be known with some degree of precision. The bestmetals for resistive heaters usually have very small positivetemperature coefficients and therefore measurement of the change inresistance by only monitoring current will be desensitized by harnessresistance and aging of numerous interconnecting components. Therefore,it becomes difficult to distinguish a change in resistance of the heatercomponent from a change in resistance of other components connected inseries.

It would be advantageous to more precisely know the resistance change ofthe heater component such that control of the temperature may beaccomplished.

BRIEF SUMMARY

In accordance with embodiments of the invention, electrical resistanceof a heated component is determined for temperature control andmonitoring. A voltage-representation signal, which is proportional to avoltage across a heater component, is received. A magnifiedvoltage-representation signal is generated by determining a differencebetween a voltage baseline offset, which represents a minimum operatingvoltage of the heater component, and the voltage-representation signal.A current-representation signal, which is proportional to an electricalcurrent passing through the heater component is received. A magnifiedcurrent-representation signal is generated by determining a differencebetween a current baseline offset, which represents a minimum operatingcurrent of the heater component, and the current-representation signal.The magnified current-representation signal is modulated to generate aresistance-representation signal that is proportional to an amount ofmodulation that makes the magnified current-representation signalapproximately equal to the magnified voltage-representation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an operating environment forembodiments of the invention.

FIG. 2 is a signal flow diagram in accordance with embodiments of theinvention.

FIG. 3 is similar to FIG. 2 and depicts additional components forincreased sensitivity of resistance measurements in accordance withembodiments of the invention.

FIG. 4 is a simplified schematic diagram in accordance with embodimentsof the invention.

FIG. 5 is similar to FIG. 4 and depicts additional components forincreased sensitivity of resistance measurements in accordance withembodiments of the invention.

FIG. 6 is a plot of oscilloscope data depicting functionality inaccordance with embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the invention are directed to determining the electricalresistance of a heater component above a minimum operating point andproviding a signal that represents this resistance in a range ofinterest as an output solution of division equivalent of a signalrepresenting voltage by a signal representing current. Current may bemeasured by precisely measuring a voltage drop across a small valueprecision resistor inside an electronics assembly, or “current-senseresistor.” This voltage drop is directly proportional to the currentflowing through the resistor. Knowledge of this current may then beexpanded upon by a precise measurement of voltage across the heatercomponent. Many systems, such as automotive electronic systems, have aminimum operating voltage, below which the system is typically lockedout of function or deactivated. It does not make good use of resourcesand measurement range to include this minimum operating point, asnothing below that value will be measured. The minimum value ofoperating voltage can be subtracted from the measured value of voltageacross the heater component, and the remainder amplified over theuseable signal range, thereby increasing the sensitivity of themeasurement to changes over a range of interest. Likewise, there will bea minimum current through the heater device at the minimum voltage ofthe system that may be subtracted from the measured current, theremainder amplified and again increasing the sensitivity of themeasurement to changes over a range of interest. This magnified voltageacross the heater component is divided by the magnified voltageproportional to the current over a range of interest to satisfy therelationship from Ohm's Law such that the resistance over a range ofinterest may now be known, in accordance with the formula R=V/I, where Ris resistance, V is voltage, and I is current. The resistance may beprovided as a voltage proportional to the duty cycle of a Pulse WidthModulated (PWM) signal. The PWM signal has a duty cycle that makes thevoltage proportional to current through the heater component equivalentto the voltage proportional to the voltage across the heater component.Embodiments of the invention generate this resistance knowledge, andfrom this knowledge, the temperature of the heated component may beestimated and then regulated based on the estimated temperature.

FIG. 1 is a diagrammatic representation of an operating environment forembodiments of the invention. In FIG. 1, a functional location ofembodiments of the invention is depicted as a division equivalent module113. A heater 110 for an automotive component, including, but notlimited to, an inductively heated fuel injector or a heated componentused for exhaust gas after treatment, references the heater component ofwhich a resistance, as a function of temperature, is to be determined.An I-sense resistor differential voltage, also referred to as heatercurrent signal 120, represents the electrical current through theI-sense resistor 122 and, therefore, through the heater 110. A currentmeasurement circuit 127 comprises the I-sense resistor 122 and adifferential voltage operational amplifier 126. A current sense resistormay be used either on the high side or the low side of the power switchor the load. Current measurement may be done with a hall sensor or withother types of magnetic sensors, such as sense coils.

A differential voltage across the heater, also referred to as heatervoltage signal 108, represents the excitation voltage directly relatedto the current flowing through the heater. The two differential voltagesare solved for Ohm's Law relation, R=V/I, using an analog or digitaldivision equivalent 113, to provide a result as a voltage-equivalentheater resistance signal 112. The analog or digital division equivalent113 may be implemented in accordance with conventional techniques, whichare known in the art, by combining operations and components including,but not limited to: summing and shift registers in digital solutions;and logarithmic, sum or difference, and antilogarithm amplification inanalog solutions. The change in resistance differential amplifier 118then finds a difference between the voltage-equivalent heater resistancesignal 112 and a resistance reference value, R-ref 124. This generates adelta, or change in resistance, or error, signal that may be brought inas an equivalent temperature rise signal 123 to a temperature controlmodule 130. This equivalent temperature rise signal 123 may beintegrated over time, which may be performed computationally or throughan analog conversion to perform the integration function, and may becompared to a temperature reference, T-ref 128. The temperature controlmodule 130 may use this comparison to determine if power should beremoved from the heater by turning off the power switch 116, representedby a MOSFET in FIG. 1, for this example. The temperature control module130 may be: a microcontroller, a digital “thermostat”, a PID(Proportional Integral Derivative) controller, or any interface thatuses the change in temperature (that is represented by the equivalenttemperature rise signal) integrated and compared to a target change intemperature, absolute temperature, or some other temperature reference.If the equivalent temperature rise signal 123 is too high, thetemperature change is too great, so the power switch 116 may bede-energized thereby turning off the heater 110. A cool-down model maythen be used to determine when to turn the heater on again. Or if acontinuous set point control strategy is used, then the power switch maybe turned on and off rapidly (or operated in a linear region like ananalog audio amplifier) to regulate the temperature to a targettemperature by repeatedly adjusting heater power.

The differential voltage across the heater 110 may be obtained by adifferential voltage measurement circuit 109, which may comprise adifferential voltage operational amplifier 114 and a pair of Kelvinconnections 104-1 and 104-2 to the heater as close to the actual heaterelectrical connections as possible. The pair of Kelvin connectionsrefers to the junction where force and sense connections are made. Theforce component is a high current carrying conductor and the sensecomponent is a parallel wire for obtaining a voltage potential at thatconnection. There are two Kelvin connections such that one conductorpair carries the current of the heater, and the other conductor pair isused for obtaining the voltage potential. The two pairs of wires may beof different size, with the current carrying pair of an appropriate sizeto minimize loss, and the voltage potential pair any reasonably smallsize for the measurement. In this way, these two pairs of wires may beused, in accordance with embodiments of the invention, to perform a fourwire measurement.

To measure the differential voltage, the load, or heater, may be one legof a Wheatstone bridge that is balanced. And then any change in the loadwould result in an unbalance of the Wheatstone bridge, and, therefore, adifferent voltage across the load. Or a resistance divider may belocated locally at the heater or load. And then the voltage from theresistance divider may be brought back to the electronics forinterpretation.

Referring to FIG. 2, the signal 214 representing current through theheater component 110 is modulated, by pulse width modulator 212, basedon the error, or difference, signal 210 between the signal 202representing voltage across the heater component 110 and a version ofthe modulated signal 206 representing current through the heatercomponent 110 after it has been low-pass filtered by low-pass filter205. The result of the modulation is to generate a signal 206representing the current that is equal to the signal 202 representingthe voltage, and the corresponding modulation duty cycle is theequivalent resistance of the heater component 110. The modulated signal206 is processed through a low pass filter 205 such that the variationof the signal due to modulation frequency is removed to present themodulation correction (not the variation of the pulse width rising andfalling signal) or, stated differently, the average of the high leveland the low level of the pulse width modulation. The difference betweenvoltage across the heater component and the modulated current throughthe heater component may be integrated, by difference integrationamplifier 208, to drive the error signal 210 to zero. Mathematically,the product of the duty cycle and the current through the heatercomponent is equal to the voltage across the heater component: I(%)=VSolving for percent duty cycle yields %=V/I, therefore R=V/I issatisfied with % equivalent to R. A resistance equivalent signal 204 isthen made available as a signal representing the duty cycle that drivesto zero the error 210 between the signal 202 representing voltage acrossthe heater component 110 and the modulated signal 206 representingcurrent through the heater component 110.

Referring to FIG. 3, in the upper right-hand corner, a voltage baselineoffset, which represents a minimum operating voltage value, and adifferential amplifier block are used in conjunction with the signalrepresenting voltage across the heater component. Similarly, in thelower right-hand corner, a current baseline offset, which represents aminimum amount of operating electrical current, and a differentialamplifier block are used in conjunction with the signal representing thecurrent through the heater component. The differential amplifier blockseach subtract the respective value for the minimum operating point fromtheir respective signals and amplify the respective residual signals tomaximize the range of the voltage available for those signals.

Referring to FIG. 4, an embodiment is described where differentialamplifier A provides the signal representing the voltage across theheater component as differential voltage A. Differential amplifier Bprovides the signal representing the current through the heatercomponent as differential voltage B, scaled by R2 and R3, as desired.The PWM (pulse width modulation) comparator modulates differentialvoltage B by a signal from the integration amplifier compared to atriangle, or saw-tooth, waveform to generate the pulse width modulation.R4 and C1 are the low pass filter that removes the variation of the highand low PWM values to yield an average voltage representing themodulated differential voltage B. The difference between this modulatedvoltage and differential voltage A, scaled by R5 and R6, is integratedby C2 to yield the error voltage provided to the PWM comparator and theoutput PWM comparator. The output PWM comparator is scaled by R-scalethrough R7 and filtered by the low pass filter, comprising R8 and C3, toyield a voltage, R-out, that represents resistance of the heatercomponent.

Referring to FIG. 5, an embodiment of the invention is described inwhich differential amplifier A provides a signal representing thevoltage across the heater component to differential amplifier C, wheredifferential amplifier C subtracts the voltage baseline offset from thesignal and amplifies the residual to provide a magnified differentialvoltage A. Differential amplifier B provides the signal representing thecurrent through the heater component to differential amplifier D, wheredifferential amplifier D subtracts the current baseline offset from thesignal and amplifies the residual to provide a magnified differentialvoltage B, scaled by R2 and R3 as desired. PWM comparator modulates themagnified differential voltage B by a signal from the integrationamplifier compared to a triangle, or saw-tooth, waveform to generate thepulse width modulation. R4 and C1 form a low pass filter that removesthe variation of the high and low PWM values to yield an average voltagerepresenting the modulated magnified differential voltage B. Thedifference between this modulated voltage and the magnified differentialvoltage A, scaled by R5 and R6, is integrated by C2 to yield the errorvoltage provided to the PWM comparator and the output PWM comparator.The output PWM comparator is scaled by R-scale through R7 and filteredby the low pass filter formed by R8 and C3 to yield a voltage, R-out,representing resistance, above a minimum operating point, of the heatercomponent.

Referring to FIG. 6, a steady state voltage of 5 volts, not shown, isprovided as differential voltage A to a circuit similar to FIG. 4, and avoltage triangle waveform of one Hertz frequency is provided asdifferential voltage B. The R-out signal is equivalent to Vc and isplotted as Va/Vb=Vc. The digital solution to Va/Vb is simultaneouslyplotted to show that embodiments of the invention generate a goodapproximation of R=V/I.

The foregoing detailed description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from thedescription of the invention, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the method makes reference to a low side semiconductor switch anda low side current sense resistor, a high side semiconductor switch orhigh side current sense resistor or any combination thereof asunderstood by those skilled in the art, will be within the scope of theinvention. An additional example is that, while a differential amplifieris used to remove the signals below a minimum operating point, any othermethod, such as a microcontroller or dedicated analog to digitalconverter integrated circuit, may be used without departing from thescope of the invention. Further subtraction or differentiation ofminimum operating point from signal may be supplanted by a digital oranother analog method without departing from the scope of the invention.It is to be understood that the embodiments shown and described hereinare only illustrative of the principles of the present invention andthat various modifications may be implemented by those skilled in theart without departing from the scope and spirit of the invention.

1. A method comprising: receiving a voltage-representation signal thatis proportional to a voltage across a heater component; generating amagnified voltage-representation signal by determining a differencebetween a voltage baseline offset, which represents a minimum operatingvoltage of the heater component, and the voltage-representation signal;receiving a current-representation signal that is proportional to anelectrical current passing through the heater component; generating amagnified current-representation signal by determining a differencebetween a current baseline offset, which represents a minimum operatingcurrent of the heater component, and the current-representation signal;and modulating the magnified current-representation signal to generate aresistance-representation signal that is proportional to an amount ofmodulation that makes the magnified current-representation signalapproximately equal to the magnified voltage-representation signal. 2.The method of claim 1, wherein the modulation is pulse-width modulation.3. The method of claim 2, wherein the resistance-representation signalis generated, at least in part, based on a signal that is proportionalto the percent duty cycle of the pulse width modulation.
 4. The methodof claim 1, wherein a difference of the resistance-representation signaland the magnified voltage-representation signal is integrated to providean error signal configured to control the modulation such that themodulation is proportional to an electrical resistance of the heatercomponent.
 5. The method of claim 1, wherein a difference of theresistance-representation signal and the magnifiedvoltage-representation signal is provided as an error signal configuredto control the modulation such that the modulation is proportional to anelectrical resistance of the heater component.
 6. The method of claim 1,wherein the resistance-representation signal is scaled such that theresistance-representation signal is proportional to the electricalresistance of the heater component.
 7. The method of claim 1, whereinthe current-representation signal is generated by a differential voltagemeasurement of a voltage across a resistor that is passing an amount ofcurrent that is, proportionally, approximately the same as an amount ofcurrent passing through the heater component.
 8. The method of claim 1,wherein the voltage-representation signal is generated by a differentialvoltage measurement of the heater component.
 9. Apparatus comprising: alow-pass filter that is configured to receive a modulated magnifiedcurrent-representation signal that is proportional to a differencebetween a current baseline offset and an electrical current passingthrough a heater component and that is configured to provide aresistance-representation signal; an amplifier configured to generate adifference error signal based on a difference between a magnifiedvoltage-representation signal, which is proportional to a differencebetween a voltage baseline offset and a voltage across a heatercomponent, and the resistance-representation signal; and a modulatorconfigured to modulate the magnified current-representation signal inresponse to the difference error signal such that theresistance-representation signal is proportional to an amount ofmodulation that makes the magnified current-representation signalapproximately equal to the magnified voltage-representation signal. 10.The apparatus of claim 9, wherein the modulator is a pulse-widthmodulator.
 11. The apparatus of claim 10, wherein the magnifiedresistance-representation signal is generated, at least in part, basedon a signal that is proportional to the percent duty cycle of thepulse-width modulator.
 12. The apparatus of claim 9, wherein theamplifier is configured to integrate the difference of the magnifiedresistance-representation signal and the magnifiedvoltage-representation such that the modulation is proportional to anelectrical resistance of the heater component.
 13. The apparatus ofclaim 9, wherein the amplifier is configured such that the modulation isproportional to an electrical resistance of the heater component. 14.The apparatus of claim 9, wherein the resistance-representation signalis scaled such that the resistance-representation signal is proportionalto the electrical resistance of the heater component.
 15. The apparatusof claim 9, wherein the current-representation signal is generated by adifferential voltage measurement of a voltage across a resistor that ispassing an amount of current that is, proportionally, approximately thesame as an amount of current passing through the heater component. 16.The apparatus of claim 9, wherein the voltage-representation signal isgenerated by a differential voltage measurement of the heater component.